1. Technical Field
This invention relates to a process for producing a cement clinker having a content of coal ash as a source of calcium in the feedstock introduced into the cement kiln; more especially the invention is concerned with such a process in which the coal ash is derived predominantly from lignite and sub-bituminous coal sources. The fly ash component of this type of coal ash is referred to as Class C.
2. Description of the Prior Art
In a cement plant, cement clinker is created at elevated temperatures in a cement kiln from cement clinker raw ingredients which travel through the kiln from a feed end to a discharge end, while passing through different processing zones at elevated temperature. These processing zones include a calcining and a clinkering or burning zone.
Cement clinker comprises various cement compounds formed from the raw ingredients, for example dicalcium silicate, tricalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite. Formation of these cement compounds requires sources of calcium, silicon, aluminium hand iron in the raw ingredients fed to the cement kiln.
The cement clinker raw ingredients include a source of calcium carbonate, usually limestone, as a source of calcium for the cement compounds of the cement clinker. The calcium carbonate is thermally decomposed to lime and carbon dioxide in the calcining zone. The carbon dioxide emissions represent a pollution problem as they exit from the kiln, the significance of which has heightened with geopolitical concerns surrounding the Kyoto Accord.
Coal ash is derived from the burning of lignite, bituminous and sub-bituminous coal in power plants; the fly ash is recovered from the exhaust gases of the coal burning plants, and bottom ash is recovered from the bottom of the boiler as a granular coal ash.
Coal ash including class C fly ash has been blended or interground with cement clinker, because of its pozzolanic nature, to produce blended cements or has been used as a pozzolanic admixture in concrete, but has not previously been considered as a source of calcium in the production of cement clinker. Moreover fly ash is formed as very fine particles and is normally utilized in that form, with a minimum of processing. The fine particles can present a handling problem. Bottom ash which can also be used as a source of calcium does not have this problem because of its larger particle sizes.
Fly ash is produced in huge tonnages and while uses have been developed for fly ash, the enormous quantities produced still present a disposal problem.
It is an object of this invention to employ coal ash as a source of calcium in cement clinker production.
It is a particular object of this invention to employ a coal ash derived predominantly from lignite and sub-bituminous coal as a source of calcium in cement clinker manufacture.
It is a further object of his invention to provide such processes with reduced emissions of carbon dioxide per unit weight of cement clinker produced.
In accordance with the invention there is provided in a method of cement clinker manufacture in which a clinker feed material containing a source of calcium carbonate is fed into a feed end of a cement kiln, the feed material is heat processed in the kiln to produce cement clinker with emission of carbon dioxide from thermal decomposition of the source of calcium carbonate and discharge of the carbon dioxide from the kiln, and cement clinker is discharged from a discharge end of the kiln, the improvement wherein a coal ash derived from burning pulverized lignite or sub-bituminous coal is included in the feed material in said kiln to replace a portion of said source of calcium carbonate in the formation of said cement clinker, with a lowering of the emission of carbon dioxide in said kiln, per unit weight of cement clinker produced.
In another aspect of the invention there is provided a method of producing cement clinker with reduced emission of carbon dioxide from a change of cement clinker raw ingredients comprising:
i)providing a rotary cement kiln having a feed end and a discharge end;
ii) establishing predetermined levels of at least calcium, silicon and aluminium for a cement clinker, based on a raw ingredient feed material comprising a source of calcium carbonate, a source of silicon and a source of aluminium,
iii) providing a raw ingredient feed material formulation comprising said sources, in which a portion of said source of calcium carbonate is replaced by a coal ash derived from lignite or sub-bituminous coal while maintaining said predetermined level of calcium;
iv) feeding said formulation into said kiln,
v) exposing said formulation to elevated temperatures in said kiln, while feeding said ingredients from said feed end towards said discharge end, to calcine said source of calcium carbonate with formation of calcium oxide and liberation of carbon dioxide, and chemically combine and integrate said calcium oxide with said sources of silicon and aluminium, and said coal ash, as a cement clinker, and
vi) discharging said cement clinker from said discharge end, whereby carbon dioxide emissions are reduced in proportion to the replacement of said source of calcium carbonate by said coal ash.